Components or products which generate electricity from light, commonly called solar panels, necessarily require one or more photovoltaic cells which essentially comprise silicon in a crystalline form. Typically, photovoltaic cells are produced from crystalline silicon wafers, which are sliced or produced from larger bricks of silicon called crystalline silicon ingots. Such photovoltaic cells, and likewise their parent crystalline silicon wafers and crystalline silicon ingots, may be either a monocrystalline (single-crystal) structure or a polycrystalline (multi-crystal) structure—creating two distinct families of photovoltaic cells, crystalline silicon wafers and crystalline silicon ingots in the photovoltaic industry.
Photovoltaic cells, crystalline silicon wafers and crystalline silicon ingots representing a monocrystalline structure are typically prepared in a process called the Czochralski process. To begin the Czochralski process, refined and reasonably pure silicon feedstock, commonly referred to as “polysilicon”, is loaded into a cylindrical, rounded bottom crucible and melted. When the polysilicon in the crucible has thoroughly melted into a molten silicon mass, the primary function of the Czochralski process commences as one skilled in the art directs machinery to dip and withdraw a “seed crystal” into/from the molten silicon mass. By slowly withdrawing (or “pulling”) the seed crystal and carefully controlling the slow cooling rate, a single-crystal ingot can be “grown” to a desired size or weight. This process can be costly and time-consuming to create crystalline silicon ingots on a per kilogram basis.
The second family of photovoltaic cells, crystalline silicon wafers and crystalline silicon ingots are polycrystalline and contain a plurality of crystal structures. This characteristic makes them slightly less efficient as a photovoltaic cell, however, in most applications the lower manufacturing costs of polycrystalline silicon wafers and polycrystalline silicon ingots more than offset the lower efficiency and thus provide the highest economic returns. Crystalline silicon ingots with a polycrystalline structure are produced by the Bridgman-Stockbarger crystal growth process, also called the “directional solidification” process.
In the directional solidification process known to those skilled in the art, a generally rectangular, flat bottom container (herein called a “mold”) is filled with polysilicon and subsequently melted under an inert atmosphere. When the polysilicon contents of the mold, called the “charge”, have thoroughly melted to a desired state of a molten silicon mass, the bottom of the mold (and thus the charge contained inside) is allowed to cool in a controlled manner. As this cooling occurs, one or more crystals nucleate and grow upward in the charge, thereby pushing impurities out of the expanding crystal microstructure. This slow cooling process of the entire molten silicon mass allows the crystals to grow to a large size.
While methods of loading polysilicon into a crucible intended for the Czochralski process are reasonably known to those skilled in the art, the present invention is instead directed toward improvements in loading and preparing a mold for the directional solidification process. The differences between the Czochralski and directional solidification processes are significant, both in terms of apparatus, processes and results.
First, crucibles utilized in the Czochralski process are cylindrical, typically 45 to 60 cm in diameter, and have a hemispherical bottom or a bottom with substantially rounded corners. These round features are necessary for optimum crystal dipping/pulling conditions. On the other hand, molds utilized for the directional solidification process tend to be rectangular (or square) with generally right angle corners, flat sides and a flat bottom—resembling a box rather than a cylinder.
It follows that the crystalline silicon ingot produced by a Czochralski crucible will be rounded or cylindrical in shape, whereas a crystalline silicon ingot produced by a directional solidification mold will be a generally rectangular ingot with generally right angle corners. This rectangular shape resembling a block results in a more efficient shape for a subsequent wafer slicing operation when manufacturing photovoltaic cells.
There are also significant contrasts between the melting phases of the Czochralski process and the directional solidification process. Crucibles utilized for the Czochralski process are heated by a furnace that directs heat to the sides of the crucible, one crucible at a time. This is necessary due to the fact that the top of a Czochralski crucible must be kept clear for the seed crystal and for the dipping/pulling apparatus. On the other hand, directional solidification molds are typically heated from the top and bottom, with multiple molds placed adjacent to one another in a large furnace for batch processing.
The cooling and crystallization phases of the Czochralski process and directional solidification process are also significantly different. Cooling of a Czochralski crucible occurs only when the crucible is effectively empty and no more silicon can be practically withdrawn at the end of a production cycle. To note, in the Czochralski process the majority of molten silicon mass is cooled as it is withdrawn and crystallized upon the seed crystal—occurring near the top surface of the molten silicon mass or outside the crucible altogether. To the contrary, directional solidification molds and the entire molten silicon mass contained within the mold are cooled from the bottom, with the crystallization phase beginning at the bottom of the molten silicon mass and traveling upwards.
Since crystallization by directional solidification forces impurities upwards, the topmost layer of a directional solidification ingot is often contaminated, whereas on the Czochralski process these corresponding impurities remain leftover in the bottom of the crucible after the ingot has been permanently withdrawn from the crucible.
Since the directional solidification process does not require the slow dipping/pulling process required of the Czochralski process, directional solidification is a quicker and more cost effective means to produce crystalline silicon ingots on a per kilogram basis. When utilizing the directional solidification process, the cost of producing a given crystalline silicon ingot by directional solidification remains largely independent of the actual weight of the polysilicon placed into the mold prior to the melting phase. It follows, therefore, that if more polysilicon can be loaded into the mold and processed for a given amount of power, time and labor, then the cost per kilogram of a crystalline silicon ingot is reduced.
However, certain factors which tend to increase the yield or weight of a given resultant ingot using directional solidification may also tend to also increase contamination, power consumption, time and labor required to process the batch. A discussion of these issues and the disclosed improved process for directional solidification is further detailed in the following paragraphs.